with a limited amount of time and audience attention in which to develop characters and tell a story, hollywood often relies heavily on stereotypes to convey a character. they cast people with a certain sex, weight, build, and even body of prior works as a shorthand way of communicating the character. of course, they could spend a few minutes out of their precious 90 to tell us that the lead character is a macho muscle man, but just casting ahhnold does the job and leaves more time for action.
i would LOVE to see a movie where every character is counter-cast against the traditional hollywood stereotypes. male actors playing women, female actors playing men, young actors playing the old, debonair actors playing low-lifes, etc. i mean, THAT's what acting's all about, isn't it? it has to be more than just happening to BE very nearly the character in some script.
but then, that would BE the topic of the movie, the whole point. there's not much room in 90 minutes for many themes and sub-themes. so a script-writer picks and chooses, and by relying on stereotypes, there's more room left to tell other stories and develop other themes.
i'm not saying it's right, i'm just saying what it is.
in this context, separate male/female categories at the oscars is just par for the course. no better or worse than what hollywood does with virtually every movie it makes.